1. Field of the Invention
The invention relates to a module for the metered provision of a liquid. The module can be installed into a tank, in order to remove liquid from the tank and to provide it in a metered manner. The module can be used, in particular, in a motor vehicle, in order to meter a liquid additive for exhaust gas purification from a supply tank into an exhaust gas treatment apparatus.
2. Related Art
Such modules can be used, in particular, for exhaust gas treatment apparatuses, in which the SCR method (SCR=selective catalytic reduction) is carried out. In the SCR method, nitrogen oxide compounds in the exhaust gas of the internal combustion engine are reduced with the aid of a reducing agent. Ammonia is usually used as reducing agent. Ammonia is often not stored directly in motor vehicles, but rather in the form of a liquid additive that can be converted into ammonia within the exhaust gas (within the exhaust gas treatment apparatus) and/or outside the exhaust gas (in a reactor which is provided for this purpose). A liquid additive used for the exhaust gas purification is urea/water solution. A 32.5% urea/water solution is available under the commercial name AdBlue® for use in the SCR method.
In the construction and the design of modules for the metered provision of liquid additives, it is to be noted that the liquid additives freeze at low temperatures. The above urea/water solution freezes, for example, at a temperature of −11° C. Low temperatures of this type can occur in motor vehicles during long stationary times in winter. In particular in the urea/water solution, a volumetric expansion occurs during freezing, which volumetric expansion can damage the module and/or the tank in which the module is installed, for example by a line or a component for delivering the liquid additive being deformed or even broken open. A module for the provision of a liquid additive should be constructed in such a way that it is not damaged during (repeated) freezing of the liquid additive. In addition, it is desirable in motor vehicles that liquid additive can already be provided briefly after the operational starting of the motor vehicle. This produces difficulties, in particular, when the liquid additive has frozen. In order to heat the liquid additive in the tank, it is known to provide a heater in the tank, by way of which heater the liquid additive can be heated. A heater of this type should be operated with energy consumption which is as low as possible.